[Xen-users] Suggestions for the xen user's manual / wiki

[Michael Heyse <mhk@xxxxxxxxxxxxxxxxx>]

Hi,

there are two issues wich cost me some time to figure out and maybe it
made sense if someone put them into the documentation and / or wiki,
because without these addressed my system ran pretty instable and wasn't
very usable. This applies to xen-3.0 and kernel 2.6.12.6-xen:


1) Networking issue

With the default bridged setup, networking was quite flaky, e.g. samba
access from dom0 to a server in domU was impossible, ssh connections
froze, and dmesg contained lots of lines like "Received packet needs 8
bytes more headroom". A short explanation is found in

http://lists.xensource.com/archives/html/xen-devel/2005-12/msg00226.html

and is apparently due to some bug in the networking code. The suggested
workaround fixed this problem for me: set the mtu to 1400 for all
network interfaces in dom0 and domU.

I'm actually suprised that this is not a FAQ.


2) Scheduling issue

The system was unusable with high CPU load in dom0, all other domains
were almost dead, and there's apparantly no comprehensive documentation
of xen's schedulers (please point out the relevant docs to me if I'm
wrong, all I found was sedf_scheduler_mini-HOWTO.txt,
xenwiki/Scheduling and `man xm`, none of which helps to understand
scheduling well enough to change its parameters). Thanks to Tim Freeman
I found an easy setup that works for me: I set the scheduling parameters
for all domains via "xm sched-sedf <domID> 0 0 0 1 1" or something
similar corresponding to example "4 domains with weights 2:3:4:2" from
the sedf_scheduler_mini-HOWTO.txt with an additional 1 as the extratime
parameter if you want to allow a domain to allocate more CPU time if
other domains are idle.


With these two issues fixed the system runs just great.

As a total xen noob I don't dare touch the wiki, but if someone more
experienced could verify that these issues are valid, adding some hints
to the documentation could help other xen beginners.

Thanks,
Michael

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